Midget fuse blowing at speed

[MrTickle]

Hi, hoping for so ideas on this one.

The 35a glass fuse from which the rev counter etc. runs on my MG Midget 1500 keeps blowing when the speed approaches 70mph.

Seem ok at lower speeds - so assuming that for some reason the alternator is generating too much output as the speed increases, or something like that.

Car has an electronic ignition conversion and coil and cables etc. all look pretty new.

Any ideas????

Thanks.

[MrTickle]

Should maybe add... When the fuse goes the car dies so it is a real PITA!

[MrTickle]

This has started to happen at slightly lower speeds now, around 50, so seems to be getting worse. Any ideas of what to try? Seems it's the Rev counter/coil circuit from googling.

[MrTickle]

Thanks Ray, will check that. Battery seems ok though and it has been doing this a while so wondering coil/rev counter issue or something amiss with the electronic ignition. Don't think it is a short as at lower speeds you can drive around quite happily with no problems.

[MrTickle]

Nice steady 14.4v right through the Rev range so alternator all ok!

[MrTickle]

Could a coil cause something like this? Could be worth just changing it to see as they are not too expensive?

[MrTickle]

Geordie MGmike said:

I've jumped over from the MGBBS to add my bit

Clarification question(s)?
1, Is the fuse that blows the one feeding the "green" circuits i.e the one that has power only when the ignition is on?
2, Are you saying the car stops when the fuse blows and won't start again until the fuse is replaced?
3, Which electronic ignition unit is fitted?

Before I get the answers I'll give you my first guess just for fun
The electronic ignition has been wired in to the wrong side of the fused ignition on circuit and is drawing too much current at higher speeds (if it's getting worse it could be failing slowly or you are using more of the other items on the circuit at this time of year)

Firstly, thanks for jumping on the thread - much appreciated.

I will have another look and see what wires are connected to the fuse. To try and be as defined as possible, the car drives along perfectly well, then as the speed builds - like I say, it used to be 70 but last time was 50 (I might have had the wipers on at the time, not 100% sure) the fuse will suddenly blow, the rev counter drops to zero instantly, but the ignition lights remain on, the car dies and coasts to a stop. Then the fuse needs replacing before it will start again - starter motor turns, ignition lights on but no firing and no rev counter movement until the fuse is replaced.

I bought the car in its current 'state' so I am unsure as to which ignition is fitted. The car and wiring, other than the electronic ignition, look very 'original' and everything looks very new under the bonnet. I will have a nose around and see if I can work out the brand of the ignition system (or check paperwork to see if it is listed there).

[MrTickle]

OK - I gave had chance to take a look in daylight and have taken a picture to assist. I think you may well be right Mike.

Here is the fuse box (the blue one is the one blowing)



Most wiring seems to be coming from the original loom with the exception of the black one with the different spade connector. That is a new wire and is connected to a red one coming out of the distributor (presumably the electronic ignition module).

Should this be connected on the other side / somewhere else?

[MrTickle]

Geordie MGmike said:

Yep connect to the white on the other side of the fuse.

A word of caution here.... the ignition circuit should not draw enough current to blow a 35A fuse on it's own (more like 3 to 4A max) but will/does have peak surge currents that may. However, when combined with your comment about it getting worse over time, I'm concerned the electronics unit may be failing and the fuse blowing is a symptom of a short circuit when "hot". Therefore, If you get ANY miss fire or the engine still dies with the unit connected as above, switch off as soon as possible and replace the ignition unit.

Best of...

Brilliant, I will keep an eye on it. Thanks again for your help!

[MrTickle]

OK I am hoping this is what you meant regarding the photos...

This is the shot from inside the dizzy with rotor arm removed for clarity:


This is the ignition unit fitted:


This is the general layout of the engine bay (just in case!)

[MrTickle]

Just to add, it looks like this is what has been fitted:

http://www.ebay.co.uk/itm/MG-Midget-1500-electroni...

[MrTickle]

Ok - wire swapped over and been out for a spin.

Good news is that the car now doesn't conk out!

Bad news, the fuse blew again.

So at least I know it's not a dodgy ignition. Must be either a short or rev counter problem.

[MrTickle]

Got a bag of 25 fuses from eBay to keep me going while I identify the culprit!

[MrTickle]

Hi Nigel,

Thanks for your input. The wires in the dizzy look a bit tight, as I must admit to took a bit of the slack off when I was following the wires. I have since pushed them through again to reduce tension.

The HT leads are routed quite well, might be just the photo. Since I have moved the electronic ignition feed to the unfused side, as per the manufacturer instructions, it has ruled out the ignition side as being a problem. Now the fuse still blows, but ignition is all good.

I would guess it has been fitted as a kit, as the dizzy, ignition, HT leads and coil all look exactly the same as that ebay link I put up. Agreed they do look pretty cheap - but at the moment, the car starts and runs very easily and smoothly. I will consider an upgrade in the future maybe.

It would appear the short is elsewhere. I noticed one of the reversing lights is out.... so that sounds like a good place to start!